Dengue is a disease that is caused by four closely related viruses that are
maintained in a human-Aedes aegypti-human cycle in most urban centers of the
tropics (Gubler, 1997). The geographic distribution of the dengue viruses and
mosquito vectors (Aedes aegypti and A albopictus) has expanded to the point
that dengue has become a major tropical urban health problem (Gubler, 1997,
1998b). Dengue is primarily an urban disease; more than half of the world's
population lives in areas of risk (Gubler, 1997, 1998b). In tropical areas of
the world, dengue transmission occurs year-round but has a seasonal peak in
most countries during months with high rainfall and humidity. Major factors
causing epidemics include population growth, rapid urbanization, lack of effective
mosquito control, and movement of new dengue virus strains and serotypes between
countries (Gubler, 1997, 1998b).

The global resurgence of dengue in recent years has resulted in increased imported
dengue and cases of local transmission in the United States and Australia. As
with malaria, the number of cases is small and sporadic (Gubler, 1989, 1997,
1998b). By contrast, Mexican states bordering the United States have had repeated
large epidemics of dengue (Gubler, 1989, 1998b; Reiter, 1997; Rawlins et al.,
1998). The difference in vulnerability may be caused by differences in living
standards and human behavior, which in the United States decrease the probability
that vector mosquitoes will feed on humans. It is unlikely that climate change
will affect these factors and cause increased epidemic dengue activity in temperate
zone developed countries.

9.7.2.1 Modeling the Impact of Climate Change on Dengue

To date, all published studies regarding evaluations of the possible impact
of global climate change on dengue transmission have involved modification of
the standard equation for vectorial capacity (VC) (Jetten and Focks, 1997; Martens
et al., 1997; Patz et al., 1998a). Temperature affects the rate of mosquito
larval development, adult survival, vector size, and gonotrophic cycle, as well
as the EIP of the virus in the vector (Focks et al., 1993a,b, 1995).

Modeling studies (Jetten and Focks, 1997; Martens et al., 1997; Patz et al.,
1998a) suggest that a warming projection of 2°C by 2100 will result in a
net increase in the potential latitudinal and altitudinal range of dengue and
an increase in duration of the transmission season in temperate locations. However,
they also ignore the complex epidemiological and ecological factors that influence
transmission of dynamics of dengue. Changes in potential transmission in areas
that currently are endemic for dengue are projected to be limited. As with malaria,
models indicate that the areas of largest change of potential transmission intensity
as a result of temperature rise are places where mosquitoes already occur but
where development of the virus is limited by temperature during part of the
year. However, these models do not incorporate demographic, societal, and public
health factors that have been responsible for eliminating dengue from temperate
areas. Transmission intensity in tropical endemic countries is limited primarily
by herd immunity, not temperature; therefore, projected temperature increases
are not likely to affect transmission significantly. Moreover, in subtropical
developed areas, where transmission is limited primarily by demographic and
societal factors, it is unlikely that the anticipated temperature rise would
affect endemicity (Gubler, 1998b).

Table 9-3: Temperature thresholds of pathogens
and vectors. Tmin is minimum temperature required for disease
transmission. Tmax for the pathogen is upper threshold beyond
which temperatures are lethal. Tmax for vectors are not provided.
Temperatures are in degrees Celsius. Note that temperatures assume optimum
humidity; vector survival decreases rapidly as dryness increases. There
is considerable variation in these thresholds within and between species
(Purnell, 1966; Pfluger, 1980; Curto de Casas and Carcavallo, 1984; Molineaux,
1988; Rueda et al., 1990).